The Moon is spirallingaway from Earth due to tidal friction, which slows Earth’s rotation. I used to think tidal braking would stop when Earth’s daysidereal day, to be specific. matched the Moon’s orbital period, as with Pluto and Charon.

Then I realized that even though this would stop lunar tidal friction, solar tidal friction wouldn’t stop until Earth’s day matches its year. Because lunar tides are about twice as strong as solar tides, lunar braking would probably finish first. That would lock Earth’s day to the Moon’s orbital period. Then that tidally locked Earth-Moon system would slow down and move away from the Sun until the Moon’s orbital period around Earth matches Earth’s orbital period around the Sun.

I think this means the Moon would eventually end up in either the Sun-Earth L1 or L2Lagrange points. Since Lagrange points are entries into the interplanetary transport network, that seems to suggest that the Moon would be more susceptible to orbital perturbations. Conceivably, the Moon could even be ejected from the solar system with much less energy than one would expect if it hadn’t settled into a Sun-Earth Lagrange point. (Right?)

Even though our Sun would probably go red giant before this process finishes, a red dwarf with less than ~80% the mass of our Sun has an estimated lifetime longer than the ~13.8 billion year age of the universe. First generation stars seem to have been dominated by hypergiants and supergiants, but their bright flames died out quickly, seeding the cosmos with metals that may have helped smaller stars form. Some red dwarfs might be ~10 billion years old, and they’re still just infants.

Furthermore, since tidal forces scale as the inverse cube of the separation distance, planets in closer orbits around red dwarfs would experience much faster tidal braking. An Earth-Moon system in the “habitable zone” of an ancient 0.3 solar mass red dwarf might have already established lunar and solar tidal locks long ago.

Since most stars in the Milky Way are red dwarfs, and surveys suggest that most red dwarfs have planets, this could be a large source of moons which have been tidally locked into Lagrange points and potentially ejected into interstellar space.

Is this potential source of rogue planets included in searches for baryonic dark matter? Even though CMBR anisotropies and abundant deuterium show that most dark matter is non-baryonic, projects like OGLE search for small fractions of baryonic dark matter like brown dwarfs using microlensing. Wikipedia claims that these searches have only excluded objects with half Earth’s mass and above, so they don’t seem to be sensitive enough to detect objects the size of our Moon drifting in interstellar space after being ejected from a red dwarf system.

Unfortunately, I don’t know enough astrophysics to know if my musings make any sense, or alternatively if they were already considered decades ago.

One Response to “The Moon’s final resting place”

Just to be clear, I’m curious to see if this new (?) ejection mechanism could “significantly” increase ejection rates over standard dynamical interactions with other planets and stars which produce some massive compact halo objects (MACHOs).

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